1. Field of the Invention
The present invention relates to a mobile communication method, a mobile communication system and a radio base station, in which an uplink packet is transmitted from a mobile station to a radio base station, at a transmission rate corresponding to a granted value notified by the radio base station.
2. Description of the Related Art
As shown in FIG. 1, in a mobile communication system employing an enhanced uplink (EUL) system defined in 3GPP, that is, a high speed uplink packet access (HSUPA) system, a mobile station UE is configured to transmit an uplink packet via an Enhanced Dedicated Physical Data Channel (E-DPDCH) at a transmission rate corresponding to a granted value (SG: Scheduling Grant) notified by a radio base station NodeB.
To be more specific, the mobile station UE is configured to select, with reference to a previously notified table 1 shown in FIG. 1, an E-DCH transport format combination identity (E-TFCI) corresponding to the SG notified by the radio base station NodeB. Then, the mobile station UE transmits the uplink packet (transport block) at a transmission rate corresponding to the E-TFCI, via the E-DPDCH.
Here, the SG (granted value) shows a ratio between a transmission power in an enhanced dedicated physical data channel (E-DPDCH) for transmitting the uplink packet, and a transmission power of an uplink dedicated physical control channel (DPCCH).
In the 3GPP, as the SG, an absolute grant (AG) that indicates the value of the SG itself or a relative grant (RG) that indicates an increase or decrease in the value of the SG (Up/Down/Hold), are defined.
In addition, the aforementioned mobile communication system is configured to guarantee a reception error of the uplink packet in the radio base station NodeB, by use of a hybrid ARQ (HARQ) retransmission control.
With reference to FIG. 2, a description will be given of operations for transmitting an uplink packet from a mobile station UE in the mobile communication system employing the EUL.
In step S1001, a mobile station UE transmits scheduling information (SI) to a radio base station NodeB, and thereby reports the amount of the uplink packet data remaining in the transmission buffer of the mobile station UE, an upper limit value of the transmission power that can be assigned to the E-DPDCH in the mobile station UE, and the like.
In step S1002, a scheduler 10 of the radio base station NodeB determines an SG (AG or RG) to be notified to each mobile station. Here, the scheduler 10 determines the SG based on the SI reported by each mobile station, the entire reception power of the radio base station NodeB, or the like.
In step S1003, the radio base station NodeB notifies the SG (AG or RG) determined in step S1002, to each mobile station UE via an E-DCH absolute grant channel (E-AGCH) or an E-DCH relative grant channel (E-RGCH).
The mobile station UE determines the SG based on the notified AG or RG and then selects an E-TFCI corresponding to the determined SG. Thereafter, in step S1004, the mobile station UE notifies, to the radio base station NodeB, the E-TFCI by use of an E-DCH dedicated physical control channel (E-DPCCH). Then, in step S1005, the mobile station UE transmits the uplink packet to the radio base station NodeB at a transmission rate corresponding to the E-TFCI, via the E-DPDCH.
Furthermore, with reference to FIG. 3, a description will be given of operations for updating a target signal to interference ratio (SIR) used in an inner loop transmission power control in a radio base station NodeB, in the mobile communication system employing the EUL.
As shown in FIG. 3, in step S2001, the radio base station NodeB determines whether the reception of the uplink packet was successful or not (the reception success or the reception failure), by performing a cyclic redundancy check (CRC) in each sub-frame (TTI: Transmission Time Interval).
When the radio base station NodeB determines that the reception of the uplink packet was successful, as shown in step S2002, the radio base station NodeB decreases the target SIR used in the current sub-frame by an amount of Δ down and then sets the decreased target SIR to be used in the next sub-frame.
On the other hand, when the radio base station NodeB determines that the reception of the uplink data was not successful, as shown in step S2003, the radio base station NodeB increases the target SIR used in the current sub-frame by an amount of Δ up and then sets the increased target SIR to be used in the next sub-frame.
In step S2004, by use of the updated target SIR, the radio base station NodeB performs the inner loop transmission power control for the transmission power of a DPCCH in the next sub-frame.
Specifically, as shown in FIG. 4, in each sub-frame, the radio base station NodeB is configured to increase the target SIR, by the amount of Δ up, to be used in the inner loop transmission power control, when the reception of the uplink packet was not successful (CRC: NG).
On the other hand, in each sub-frame, the radio base station NodeB is configured to decrease the target SIR, by the amount of Δ down, to be used in the inner loop transmission power control, when the reception of the uplink packet was successful (CRC: OK).
In this regard, by setting the relationship between Δ up and Δ down as follows (formula 1), the reception quality (BLER: Block Error Rate) of the uplink packet in the radio base station NodeB can be made closer to the target BLERΔ down=−Δ up×(target BLER)/(1−target BLER)  (formula 1)
In the example of FIG. 4, the target BLER is “⅛,” and the radio base station NodeB increases the target SIR only by the amount of Δ up, when the reception of the uplink packet is not successful in the TTI #1.
As a result, the target SIR becomes higher, so that the receptions of the uplink packet at the radio base station NodeB are successful seven times in a row from the TTIs #2 to #8.
Here, the radio base station NodeB decreases the target SIR by the amount of Δ down every time the reception of the uplink packet is successful. Accordingly, the target SIR becomes low in the TTI #9. Thus, the reception of the uplink packet is not successful in the TTI #9, and then, the radio base station NodeB increases the target SIR only by the amount of Δ up.
However, there has been a problem in the conventional mobile communication system employing the EUL, that is, a reception success rate in the radio base station NodeB is not guaranteed, although the transmission rate at a mobile station UE is guaranteed.